Study On The CO-SCR Performance Of Mn-based Catalysts And Their Resistance To H₂O/SO₂

With the development of the industrial age,the excessive consumption of various energy sources has produced many air pollutants.Among them,NOx is one of the most important air pollutants.The environmental pollution problem needs to be solved urgently.Researchers have studied many NOx reduction processes in recent decades.NH3-SCR is the most effective.However,NH3 is used as a reducing gas to participate in the entire chemical reaction.It is easy to cause ammonia escape,ammonia leakage or more serious secondary pollution problems.Due to the shortcomings of NH3-SCR technology,extensive researches have been carried out on the reducing agent in the reduction reaction.CO is cheap and easy to obtain.It is friendly to NO reduction equipment,and has no secondary pollution.It has become a research hotspot in recent years.Whether in NH3-SCR or CO-SCR,the catalyst has no excellent resistance to H2O and SO2 in the flue gas atmosphere.Therefore,starting from the type of catalyst,this article studies the resistance of metal oxide catalysts in CO-SCR.The reaction mechanism of sulfur and water resistance is compared with the reaction mechanism of the NH3-SCR catalyst with better sulfur and water resistance,so as to further study the resistance of this type of catalyst.At the same time,based on the first principles,DFT molecular simulation calculation is used to simulate the reaction mechanism of the catalyst,and theoretically demonstrates the conclusions in the experiment.First of all,this article conducted an in-depth study on the H2O resistance of metal oxide catalysts.The MnM(M=Ce,Cu,Co)catalyst was prepared by the co-precipitation method,and the activity of the three catalysts was tested on a fixed bed reactor.The order ofthe water resistance ofthe three catalysts for CO-SCR is as follows:MnCeOx>MnCoOx>MnCuOx.At the same time,the experimental characterization and simulation analysis of the three catalysts proved that the presence of H2O in the flue gas atmosphere inhibits the NO reduction of the catalyst under low temperature conditions.This is because water molecules will preempt the active sites of NO and CO under low temperature conditions.With the temperature increasing,on the one hand,the H2O is removed and the active site is increased,on the other hand,the Brost acid site is generated,which can adsorb CO and promote the reaction.Secondly,this article conducted an in-depth study on the SO2 resistance of metal oxide catalysts.The MnM(M=Co,Cu,Fe)Ce catalyst was prepared by co-precipitation method,and the NO reduction of these three catalysts was tested on a fixed bed reactor.After 200ppm SO2 was introduced,the NO reduction efficiency of the catalyst decreased with the increase of time.After 6h,the three catalysts were completely deactivated,and the NOx conversion rate of the catalyst was less than 10%.However,after stopping the introduction of SO2 for 2h,the catalytic activity of MnCoCe and MnFeCe can be recovered.The order of the sulfur resistance performance of the three catalysts for CO-SCR is:MnCoCe>MnFeCe>MnCuCe.At the same time,the three catalysts were characterized and simulated.The three catalysts have competitive adsorption of NO and SO2,and the adsorption capacity of NO in the MnCoCe catalyst is stronger than the adsorption capacity of SO2 at high temperatures.When NO is adsorbed,it will be formed.There are more nitrate species than MnFeCe and MnCuCe,so the sulfur resistance of MnCoCe is the best DFT simulation calculation results show that at low temperature,the adsorption energy of SO 2 molecules is the largest when they are adsorbed on Co atoms.The adsorption energy is-1.62eV.The adsorption energy is larger than that of NO and CO.It means that if there is SO2 in the flue gas,SO2 will preempt the two gases.The adsorption sites of NO and CO will be inhibited,so the efficiency of CO-SCR will be reduced.As the temperature increases,NO molecules will form N2O2 dimers,with an adsorption energy of-2.732eV,which is significantly higher than the adsorption energy of SO2.N2O2 drives the SO2 adsorbed on Co atoms out of the adsorption sites,so the increase in temperature can not only improve the NO reduction efficiency of MnCoCe,but also improve the anti-sulfur effect.Finally,this article conducted an in-depth study on the SO2 and H2O resistance of VCe and MnCoCe catalysts in NH3-SCR and CO-SCR,respectively.Three characterization methods,BET,XRD and XPS,are used to study the sulfur and water resistance of the catalyst from the internal structure and valence changes ofthe catalyst At the same time,the DFT molecular simulation calculation is used to simulate its sulfur and water resistance mechanism.For the VCe catalyst,its sulfur and water resistance performance is better than that ofthe MnCoCe cataly st.This is because after the catalyst adsorbs H2O and SO2,the active sites in the VCe can still function.At high temperatures,NH4HSO4 will react with NO and decompose,releasing active sites.For the MnCoCe catalyst,the DFT simulation calculation results show that when H2O and SO2 are present on the surface of the catalyst,the two are prone to react to form HSO3-,which has a great adsorption energy and is tightly bound to the metal on the catalyst surface.It is difficult for the gas in the CO-SCR reaction to react with the desorption from the catalyst surface.At the same time,when SO2 and H2O react,it will consume the OH-that is about to be formed on the catalyst surface,and the subsequent adsorption of CO and NO will be weakened,which will reduce the activity of the CO-SCR reaction.Thus,when H2O and SO2 existat the same time,the sulfur and water resistance of the catalyst are not ideal.In summary,in the study on the CO-SCR performance of Mn-based catalysts and their resistance to H2O/SO2,the mechanism of the water and sulfur resistance of the series of catalysts is fully analyzed through the experiment and simulation double-layer method which provides a good idea for the next step to find a catalyst with good water and sulfur resistance.